The present invention relates to a method for absorbing formaldehyde from gaseous mixtures that contain it and to a method for regenerating the absorption compound used.
A study conducted by the EPA (Environmental Protection Agency) of the United States and lasting over 10 years has shown that some atmospheric pollutants concentrate more in enclosed environments than outdoors, with a negative effect on our state of health.
In particular, it has been observed that building materials and furniture can release toxic chemicals for long periods of time.
Formaldehyde (H2Cxe2x95x90O) is the most important of the gases that pollute indoor environments and is a component of the atmosphere that forms as a stable intermediate in the oxidation of methane or other volatile organic compounds. While its outdoor concentration is generally too low to be important (approximately 0.01 ppm in built-up areas, except in episodes of photochemical smog), the indoor level of formaldehyde is often greater by orders of magnitude (on average 0.1 ppm, but sometimes  greater than 1 ppm).
The main sources of emission of this gas in homes are cigarette smoke and the synthetic resins and foams based on urea-formaldehyde used as insulating materials. These resins and foams, in certain conditions such as high humidity, both release high rates of formaldehyde for long periods of time.
One of the main uses of formaldehyde is in the production of synthetic resins. These synthetic resins, such as urea-formaldehyde, melamine-formaldehyde, urea-melamine-formaldehyde, phenol-resorcinol-formaldehyde, have long been used as adhesives in the wood industry to produce panels of particleboard and plywood. The resins are capable of releasing formaldehyde for long periods of time.
Although formaldehyde has been used in the past mainly as disinfectant for sanitary environments, clothes, garments and utensils of sick individuals, it is a substance that in addition to being a known allergen is a suspected mutagen and carcinogen for human beings (information in this regard is provided in the circular letter dated 22 Jun. 1983, no. 57, of the Ministry of Health, entitled xe2x80x9cUsi della formaldeide, rischi connessi alle possibili modalità di impiegoxe2x80x9d [Uses of formaldehyde, risks linked to its possible methods of use].
To avoid damage to health, a series of European standards (EN 120, EN 717-2, EN 717-1) is currently in force and lists xe2x80x9cthe directives for use of particleboard panels in enclosed spacesxe2x80x9d, which set the maximum allowed limits for release of formaldehyde (Table 1).
The methods for eliminating formaldehyde cited in patent literature mainly relate to solutions of functionalized phenols to be used in a mixture with the formaldehyde-containing synthetic resins and capable of absorbing the released formaldehyde. Information is also provided on formaldehyde-absorbing materials, supported by porous silicates and characterized by active functional groups such as amines, thiols and sulfonic acid, JP 11228954A.
These methods for eliminating formaldehyde lead to products in which the formaldehyde is bonded chemically to the absorber in a manner similar to what occurs between formaldehyde and the other monomers used for the resins. The elimination of the formaldehyde trapped by the absorber is possible only by combustion of the formaldehyde-absorber system and with environmental consequences that are similar to those encountered in the combustion of said resins. The present invention provides an alternative way of trapping the formaldehyde within a matrix constituted by inorganic material and for converting the formaldehyde into environmentally compatible products by virtue of low-temperature ( greater than 200xc2x0 C.) thermal treatments.
The aim of the present invention is to eliminate the drawbacks noted above of known methods for eliminating formaldehyde from gaseous mixtures that contain it, which allows to eliminate the formaldehyde and to further convert completely the formaldehyde into fully environmentally compatible products.
An object of the present invention is to provide a method for absorbing formaldehyde that has a low cost.
Another object of the present invention is to provide a method for absorbing formaldehyde that uses a regenerable absorption agent and a method for regenerating said absorption agent.
This aim and these and other objects are achieved by the method according to the present invention for absorbing formaldehyde from gaseous mixtures that contain it, said method comprising the step of contacting gaseous mixtures that contain formaldehyde with absorption compounds constituted by mixed oxides of copper chosen among Ca4+xCu5O10, where 0xe2x89xa6xxe2x89xa61, derivatives thereof by isovalent and/or heterovalent substitutions, and mixtures thereof. Preferably, in the method according to the invention absorption occurs in the presence of oxygen and/or water vapor.